The correct option is O2.
Explanation
According to Molecular Orbital Theory (MOT), the magnetic nature of a molecule is determined by the distribution of electrons in its molecular orbitals. A molecule is paramagnetic if it possesses one or more unpaired electrons. If all electrons are paired, the molecule is diamagnetic.
Analysis of Options
- (a) O2 (Oxygen): The oxygen molecule has 16 electrons. Based on the energy level diagram for molecules with more than 14 electrons, the electronic configuration is:
σ1s2 σ1s2 σ2s2 σ2s2 σ2pz2 (π2px2 = π2py2) (π2px1 = π2py1).
The presence of two unpaired electrons in the degenerate π (antibonding) orbitals makes O2 paramagnetic. - (b) N2 (Nitrogen): The nitrogen molecule has 14 electrons. Its configuration is:
σ1s2 σ1s2 σ2s2 σ2s2 (π2px2 = π2py2) σ2pz2.
All electrons are paired, making it diamagnetic. - (c) F2 (Fluorine): The fluorine molecule has 18 electrons. Its configuration fills the π orbitals completely:
... (π2px2 = π2py2).
All electrons are paired, making it diamagnetic. - (d) Ne2 (Neon): Theoretically possessing 20 electrons, the bond order is zero (10 bonding electrons minus 10 antibonding electrons), meaning the molecule does not exist stably. However, theoretically, all electrons would be paired.
Key Takeaway: Oxygen (O2) is paramagnetic due to the presence of two unpaired electrons in its antibonding π molecular orbitals, a feature uniquely explained by Molecular Orbital Theory.